Mar 30, 2015
Early Efforts on Dating the Age of the Earth
James Ussher (mid 1600's)
Georges Buffon (mid 1700’s)
Lord Kelvin (late 1800's)
75,000 yr
100 m.y.
Charles Walcott (1893)75 m.y.
4004 B.C.
II. Constancy of Natural Laws
Catastrophism
Georges Cuvier (late 1700’s)
Uniformitarianism
James Hutton (late 1700's)
Early Efforts on Dating the Age of the Earth
Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).
Radiometric Dating of Rocks
Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.
Absolute Dating of Rocks
The main method used to get the exact age of a rock is called radioactive or radiometric dating. This method studies the decay rates of radioactive isotopes in order to determine the age of the rock.
But First What is an atom?
What is an isotope?
Fundamental unit of matter made up of subatomic particles
Electrons
Nucleus
ATOMS
NeutronsProtons
}NucleusProtons (positive charge)
Electrons (negative charge)
Neutrons (no electrical charge)
ISOTOPES
Carbon-12 Carbon-13 Carbon-14
98.9 %6 protons6 neutrons
1.1 %6 protons7 neutrons
< 0.1 %6 protons8 neutrons
12C6
13C6
14C6
Types of Radioactive Decay
alpha particle
Parent Nucleus Daughter Nucleus
alpha decay
Changes in atomic number and atomic mass number
Atomic Number = - 2
Atomic Mass Number = - 4
4H2
ALPHADECAY
238U 92
234Th 90
92 protons
146 neutrons90 protons
144 neutrons
protonsneutronselectrons
Alpha decay occurs when the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a helium nucleus is emitted.
Types of Radioactive Decay
beta particle
Parent Nucleus Daughter Nucleus
beta decay
Changes in atomic number and atomic mass number
Atomic Number = + 1
Atomic Mass Number = 0
BETADECAY
137Cs 55
137Ba 56
55 protons
82 neutrons56 protons
81 neutrons
electron
e -protonsneutronselectrons
Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted.
p +
Types of Radioactive Decay
electron
capture
Parent Nucleus Daughter NucleusAtomic Number = - 1
Atomic Mass Number = 0
ELECTRONCAPTURE
204Po 84
204Bi 83
84 protons
120 neutrons83 protons
120 neutrons
e -protonsneutronselectrons
Electron capture also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron.
Changes in atomic number and atomic mass number
U238 Pb206
Series
This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).
Radioactive Dating
• During radioactive decay, the original material or “parent material” decays at a set rate into the new material or “daughter material”
• As the number of parent material decreases, the number of daughter material increases.
• The amount of time that it takes for exactly one-half of the parent to turn into daughter is known as a half-life.
• For example – the half-life of Carbon-14 is 5,700 years. Carbon-14 is the parent material and Nitrogen-14 is the daughter material it decays into.
• Therefore, this means that it will take 5,700 years for one-half of the Carbon-14 to turn into Nitrogen-14.
Radioactive Dating
• So, why does Radioactive Dating allow geologist to get absolute dates?
• The half-lives of radioactive isotopes are set in stone and DO NOT change.
• For example, the half-life of Carbon-14 is always 5,700 years, no matter what.
• A table of some common half-lives are given in the next slide.
Radioactive Decay RatesRadioactive
Isotope Disintegration Half-Life
Carbon-14 14C 14N 5.7 x 103 years
Uranium-238238U 206Pb 4.5 x 109 years
Potassium-4040K
40Ar40Ca
1.3 x 109 years
Iodine-131
Cobalt-60
8.02 days131I
60Co 5.3 years
131Xe
60Ni
Half Life
Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb
How can we tell age based on the number of parent isotopes?
Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.
U238
Po218
Parent Isotope
Daughter Isotope
Half Life
Half-Life = 10 seconds
Half-LifeTime (s) Parent Daughter
0
1
2
3
4
5
0
10
20
30
40
50
100 0
50 50
25 75
12.5 87.5
6.25 93.75
3.125 96.875
(100)
PARENT=1,000,000DAUGHTER=0
Start with a million parent atoms…
100
50
25
12.5
6.253.13
PARENT=500,000DAUGHTER=500,000
1
1 Half-Life
100
50
25
12.5
6.253.13
PARENT=250,000DAUGHTER=750,000
1 2
2 Half-Lives
100
50
25
12.5
6.253.13
PARENT=125,000DAUGHTER=875,000
1 2 3
3 Half-Lives
100
50
25
12.5
6.253.13
PARENT=62,500DAUGHTER=937,500
1 2 3 4
100
50
25
12.5
6.25
4 Half-Lives
3.13
PARENT=31,250DAUGHTER=968,750
1 2 3 4 5
5 Half-Lives
100
50
25
12.5
6.253.13
PARENT=15,625DAUGHTER=984,375
1 2 3 4 5 6
6 Half-Lives
100
50
25
12.5
6.253.13
1 2 3 4 5 6Number of parent isotopes
Number of daughter isotopes
2.3 half-lives
20%
100
50
25
12.5
6.253.13
Example #1
If a sample has 20% 14C, how old is the sample?Half-Life 14C = 5,700 years
1 2 3 4 5 6Number of parent isotopes
Number of daughter isotopes
3.6 half-lives
8%
100
50
25
12.5
6.253.13
Example #2
A sample is found to have 131I at a concentration of 8%.How old is the sample?Half-Life 131I = 8 days
Let’s Try Some More…
Parent Daughter
1
1/2
1/4
1/8
1/16
0
1/2
3/4
7/8
15/16
1st half-life
2nd half-life
3rd half-life
4th half-life
C-14 N-14
100%
50%
25%
12.5%
6.25%
0 %
50%
75%
87.5%
93.75%
1st half-life
2nd half-life
3rd half-life
4th half-life
Once Again
Radioactive Dating
• The smaller the half-life, the less useful the radioactive isotope is for dating really old stuff.
• For example, Carbon-14 half life is only 5,700 years. Therefore, it can only accurately date rocks no older than about 100,000 years old.
• Therefore, to date rocks that formed back at the beginning of the Earth, 4.6 billion years ago, we would use an isotope with a much larger half-life.
• Since Uranium-238 half-life is 4.5 billion years, anything containing it has undergone 1 half-life since the Earth first formed!